Following is a background sketch for a potential campaign.
The rapture has come and gone. Any chosen departed. From below came the Adversary, no longer constrained to the Dungeon, to claim tyrannical dominion of what remained.
Though eternal, hubris afflicts the Adversary. Clever magic applied with savvy and sacrifice casts the Adversary back down into the Dungeon. Even the Adversary is part of existence and so subject to fundamental relations properly deployed.
So organized, the world staggers through cycles of evil and reprieve. When imprisoned, the Adversary cultivates temptations and treasures below. Invitation from those above alone is how the Adversary can return.
Inevitably, memories of past horrors fade and once vigilant guardians become complacent. Some come to doubt the existence of the Adversary below and ridicule the old rituals. Beliefs notwithstanding, opening the Dungeon’s gates requires only will and black magic.
Though deep seals prevent the Adversary from emerging fully even once the gates fail, lesser monsters are not similarly constrained. First as a trickle, then as a torrent, they come, messengers proclaiming by their rampage the Adversary’s impending arrival.
Again the gates are open. Strongholds stoke sentinel bonfires to keep the monsters at bay. Danger challenges the fellowship of humanity as alliances agreed in times of plenty crumble. As armies are tender fodder for greater monsters, only infiltration has any hope of success in reaching the inner sanctum. From the Adversary’s broken body renewed Dungeon gates may be fashioned.
To close the doors and renew all seals, venture below and defeat the Adversary. Or, seek wicked wages by breaking the seals to earn the Adversary’s acclaim and gratitude, inaugurating a new long night.